Method and system for detecting whether an acoustic event has occured along a fluid conduit

ABSTRACT

Methods, systems, and techniques for determining whether an acoustic event lias occurred along a fluid conduit iliat lias acoustic sensors positioned along its length. For each of the sensors, a processor is used to determine a linear relationship between a measured acoustic signal measured using the sensor and a white noise acoustic source located along a longitudinal segment of the fluid conduit overlapping the sensor. From the linear relationship, the processor determines an acoustic path response that includes an acoustic response of the longitudinal segment and an acoustic source transfer function dial transforms the white noise acoustic source. Over time, variations in the acoustic path responses and/or acoustic source transfer functions are monitored. When the event threshold is exceeded, the acoustic event is identified as having occurred along the longitudinal segment corresponding to the acoustic path response or acoustic source transfer function that varied in excess of the event threshold.

TECHNICAL FIELD

The present disclosure is directed at methods, systems, and techniques for detecting whether an acoustic event has occurred along a fluid conduit such as a pipeline, well casing, or production tubing.

BACKGROUND

Pipelines and oil and gas wells are examples of conduits that are used to transport liquids or gases (collectively, “fluids”) which, if leaked, could cause environmental damage. In the example of pipelines, the fluid may comprise oil. In the example of an oil well, the fluid may comprise liquid production fluid or be gaseous, such as when casing vent flow or gas migration occurs. Accordingly, in certain circumstances it may be desirable to monitor fluid conduits to determine whether a leak or other event potentially relevant to the integrity of the conduit has occurred.

SUMMARY

According to a first aspect, there is provided a method for determining whether an acoustic event has occurred along a fluid conduit having acoustic sensors positioned therealong. The method comprises determining, using a processor and for each of the sensors, a linear relationship between a measured acoustic signal measured using the sensor and a white noise acoustic source located along a longitudinal segment of the fluid conduit overlapping the sensor; and from the linear relationship, an acoustic path response and an acoustic source transfer function that transforms the white noise acoustic source. The method further comprises monitoring over time variations in one or both of the acoustic path responses and acoustic source transfer functions; determining whether at least one of the variations exceeds an event threshold; and when at least one of the variations exceeds the event threshold, attributing the acoustic event to one of the sensors corresponding to the acoustic path response or acoustic source transfer function that varied in excess of the event threshold.

The processor may attribute the acoustic event to the one of the sensors for which the variation most exceeds the event threshold.

The acoustic event may comprise one of multiple acoustic events, and wherein the processor attributes one of the acoustic events to each of the sensors for which the variation exceeds the event threshold.

The acoustic path response may comprise an acoustic response of the longitudinal segment and the acoustic event may be identified as having occurred along the longitudinal segment corresponding to the sensor to which the acoustic event is attributed.

For each of the channels, the processor may determine the linear relationship between the measured acoustic signal, the white noise acoustic source located along the longitudinal segment, and white noise acoustic sources located along any immediately adjacent longitudinal segments.

Each element of the linear relationship may be a parameterized transfer function that is parameterized using a finite impulse response structure.

The processor may determine the acoustic path responses and acoustic source transfer functions by factoring the linear relationship using a linear regression, wherein the linear regression may be factored into a first array of parameterized transfer functions for determining the acoustic path responses and a second array of parameterized transfer functions for determining the acoustic source transfer functions.

Each of the first and second arrays may be parameterized using a finite impulse response structure.

The method may further comprise, prior to monitoring variations in one or both of the acoustic path responses and acoustic source transfer functions, refining the one or both of the acoustic path responses and acoustic source transfer functions using weighted nullspace least squares.

The method may comprise determining a confidence bound for each of two of the acoustic path responses or two of the acoustic source transfer functions; from the confidence bounds, determining a statistical distance between the two of the acoustic source responses or the two of the acoustic source transfer functions; comparing the statistical distance to the event threshold; and identifying the acoustic event as having occurred when the statistical distance exceeds the event threshold.

The method may further comprising dividing the measured acoustic signal into blocks of a certain duration prior to determining the linear relationship.

Each of the longitudinal segments may be delineated by a pair of fiber Bragg gratings located along an optical fiber and tuned to substantially identical center wavelengths, and the method may further comprise optically interrogating the optical fiber in order to obtain the measured acoustic signal.

The optical fiber may extend parallel to the fluid conduit.

The optical fiber may be wrapped around the fluid conduit.

The optical fiber may be within a fiber conduit laid adjacent the fluid conduit.

The fluid conduit may comprise a pipeline.

According to another aspect, there is provided a system for detecting whether an acoustic event has occurred along a fluid conduit longitudinally divided into measurements channels. The system comprises an optical fiber extending along the conduit and comprising fiber Bragg gratings (FBGs), wherein each of the measurement channels is delineated by a pair of the FBGs tuned to substantially identical center wavelengths; an optical interrogator optically coupled to the optical fiber and configured to optically interrogate the FBGs and to output an electrical measured acoustic signal; and a signal processing unit. The signal processing unit comprises a processor communicatively coupled to the optical interrogator; and a non-transitory computer readable medium communicatively coupled to the processor, wherein the medium has computer program code stored thereon that is executable by the processor and that, when executed by the processor, causes the processor to perform the method of any of the foregoing aspects or suitable combinations thereof.

The optical fiber may extends parallel to the fluid conduit.

The optical fiber may be wrapped around the fluid conduit.

The system may further comprise a fiber conduit adjacent the fluid conduit, wherein the optical fiber extends within the fiber conduit.

The fluid conduit may comprise a pipeline.

According to another aspect, there is provided a non-transitory computer readable medium having stored thereon computer program code that is executable by a processor and that, when executed by the processor, causes the processor to perform the method of any of the foregoing aspects or suitable combinations thereof.

This summary does not necessarily describe the entire scope of all aspects. Other aspects, features and advantages will be apparent to those of ordinary skill in the art upon review of the following description of specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which illustrate one or more example embodiments:

FIG. 1A is a block diagram of a system for determining whether an acoustic event has occurred along a fluid conduit, which includes an optical fiber with fiber Bragg gratings (“FBGs”) for reflecting a light pulse, according to one example embodiment.

FIG. 1B is a schematic that depicts how the FBGs reflect a light pulse.

FIG. 1C is a schematic that depicts how a light pulse interacts with impurities in an optical fiber that results in scattered laser light due to Rayleigh scattering, which is used for distributed acoustic sensing (“DAS”).

FIG. 2 depicts a pipeline laying adjacent to a fiber conduit, according to one example embodiment.

FIGS. 3 and 4 depict block diagrams of a model for acoustic propagation along a pipeline, according to additional example embodiments.

FIG. 5 depicts a test setup used to validate a method for determining whether an event has occurred along a fluid conduit, according to another embodiment.

FIGS. 6-9 depict experimental results obtained using the test setup of FIG. 5.

FIG. 10 depicts a method for determining whether an event has occurred along a fluid conduit, according to another embodiment.

DETAILED DESCRIPTION

As used herein, “acoustics” refer generally to any type of “dynamic strain” (strain that changes over time). Acoustics having a frequency between about 20 Hz and about 20 kHz are generally perceptible by humans. Acoustics having a frequency of between about 5 Hz and about 20 Hz are referred to by persons skilled in the art as “vibration”, and acoustics that change at a rate of <1 Hz, such as at 500 μHz, are referred to as “sub-Hz strain”; as used herein, a reference to “about” or “approximately” a number or to being “substantially” equal to a number means being within +/−10% of that number.

When using acoustics to determine whether an event, such as a pipeline leak, has occurred, it may be desirable to distinguish between different types of events that generate different sounds, where “different” refers to a difference in one or both of acoustic intensity and frequency. For example, when the equipment being monitored is a buried oil pipeline, it may be any one or more of a leak in that pipeline, a truck driving on the land over that pipeline, and a pump operating near the pipeline that are generating a sound. However, of the three events, it may only be the leak that requires immediate attention. Similarly, when monitoring a well, it may be one or both of pumping equipment and an instance of casing vent flow that generate a sound. Again, while the casing vent flow may require remediation, the standard operation of pumping equipment does not.

The embodiments described herein are directed at methods, systems, and techniques for detecting whether an acoustic event has occurred along a fluid conduit such as a pipeline. Optical interferometry using fiber Bragg gratings (“FBGs”), as described in further detail with respect to FIGS. 1A-1C, is used to measure acoustics. In some of the embodiments described herein, a processor determines a measured acoustic signal using optical interferometry and from that measured acoustic signal determines whether a particular event, such as a pipeline leak, has occurred.

Referring now to FIG. 1A, there is shown one embodiment of a system 100 for fiber optic sensing using optical fiber interferometry. The system 100 comprises an optical fiber 112, an interrogator 106 optically coupled to the optical fiber 112, and a signal processing device (controller) 118 that is communicative with the interrogator 106. While not shown in FIG. 1A, within the interrogator 106 are an optical source, optical receiver, and an optical circulator. The optical circulator directs light pulses from the optical source to the optical fiber 112 and directs light pulses received by the interrogator 106 from the optical fiber 112 to the optical receiver.

The optical fiber 112 comprises one or more fiber optic strands, each of which is made from quartz glass (amorphous SiO₂). The fiber optic strands are doped with a rare earth compound (such as germanium, praseodymium, or erbium oxides) to alter their refractive indices, although in different embodiments the fiber optic strands may not be doped. Single mode and multimode optical strands of fiber are commercially available from, for example, Corning® Optical Fiber. Example optical fibers include ClearCurve™ fibers (bend insensitive), SMF28 series single mode fibers such as SMF-28 ULL fibers or SMF-28e fibers, and InfiniCor® series multimode fibers.

The interrogator 106 generates sensing and reference pulses and outputs the reference pulse after the sensing pulse. The pulses are transmitted along optical fiber 112 that comprises a first pair of FBGs. The first pair of FBGs comprises first and second FBGs 114 a,b (generally, “FBGs 114”). The first and second FBGs 114 a,b are separated by a fiber optic sensor 116 that comprises a segment of fiber extending between the first and second FBGs 114 a,b. The length of the sensor 116 varies in response to an event (such as an acoustic event) that the optical fiber 112 experiences. Each fiber segment between any pair of adjacent FBGs 114 with substantially identical center wavelengths is referred to as a “sensor” 116 of the system 200. The system 200 accordingly comprises multiple sensors 116, each of which is a distributed sensor 116 that spans the length of the segment between the adjacent FBGs 114. An example sensor length is 25 m. In the depicted embodiment, the FBGs 114 are consistently separated by, and the sensors 116 accordingly each have a length of, 25 m; however, in different embodiments (not depicted) any one or more of the sensors 116 may be of different lengths.

The light pulses have a wavelength identical or very close to the center wavelength of the FBGs 114, which is the wavelength of light the FBGs 114 are designed to partially reflect; for example, typical FBGs 114 are tuned to reflect light in the 1,000 to 2,000 nm wavelength range. The sensing and reference pulses are accordingly each partially reflected by the FBGs 114 a,b and return to the interrogator 106. The delay between transmission of the sensing and reference pulses is such that the reference pulse that reflects off the first FBG 114 a (hereinafter the “reflected reference pulse”) arrives at the optical receiver 103 simultaneously with the sensing pulse that reflects off the second FBG 114 b (hereinafter the “reflected sensing pulse”), which permits optical interference to occur.

While FIG. 1A shows only the one pair of FBGs 114 a,b, in different embodiments (not depicted) any number of FBGs 114 may be on the fiber 112, and time division multiplexing (“TDM”) (and optionally, wavelength division multiplexing (“WDM”)) may be used to simultaneously obtain measurements from them. If two or more pairs of FBGs 114 are used, any one of the pairs may be tuned to reflect a different center wavelength than any other of the pairs. Alternatively a group of multiple FBGs 114 may be tuned to reflect a different center wavelength to another group of multiple FBGs 114 and there may be any number of groups of multiple FBGs extending along the optical fiber 112 with each group of FBGs 114 tuned to reflect a different center wavelength. In these example embodiments where different pairs or group of FBGs 114 are tuned to reflect different center wavelengths to other pairs or groups of FBGs 114, WDM may be used in order to transmit and to receive light from the different pairs or groups of FBGs 114, effectively extending the number of FBG pairs or groups that can be used in series along the optical fiber 112 by reducing the effect of optical loss that otherwise would have resulted from light reflecting from the FBGs 114 located on the fiber 112 nearer to the optical source 101. When different pairs of the FBGs 114 are not tuned to different center wavelengths, TDM is sufficient.

The interrogator 106 emits laser light with a wavelength selected to be identical or sufficiently near the center wavelength of the FBGs 114 that each of the FBGs 114 partially reflects the light back towards the interrogator 106. The timing of the successively transmitted light pulses is such that the light pulses reflected by the first and second FBGs 114 a,b interfere with each other at the interrogator 106, and the optical receiver 103 records the resulting interference signal. The event that the sensor 116 experiences alters the optical path length between the two FBGs 114 and thus causes a phase difference to arise between the two interfering pulses. The resultant optical power at the optical receiver 103 can be used to determine this phase difference. Consequently, the interference signal that the interrogator 106 receives varies with the event the sensor 116 is experiencing, which allows the interrogator 106 to estimate the magnitude of the event the sensor 116 experiences from the received optical power. The interrogator 106 digitizes the phase difference and outputs an electrical signal (“output signal”) whose magnitude and frequency vary directly with the magnitude and frequency of the event the sensor 116 experiences.

The signal processing device (controller) 118 is communicatively coupled to the interrogator 106 to receive the output signal. The signal processing device 118 includes a processor 102 and a non-transitory computer readable medium 104 that are communicatively coupled to each other. An input device 110 and a display 108 interact with the processor 102. The computer readable medium 104 has encoded on it computer program code to cause the processor 102 to perform any suitable signal processing methods to the output signal. For example, if the sensor 116 is laid adjacent a region of interest that is simultaneously experiencing acoustics from two different sources, one at a rate under 20 Hz and one at a rate over 20 Hz, the sensor 116 will experience similar strain and the output signal will comprise a superposition of signals representative of those two sources. The processor 102 may apply a low pass filter with a cutoff frequency of 20 Hz to the output signal to isolate the lower frequency portion of the output signal from the higher frequency portion of the output signal. Analogously, to isolate the higher frequency portion of the output signal from the lower frequency portion, the processor 102 may apply a high pass filter with a cutoff frequency of 20 Hz. The processor 102 may also apply more complex signal processing methods to the output signal; example methods include those described in PCT application PCT/CA2012/000018 (publication number WO 2013/102252), the entirety of which is hereby incorporated by reference.

FIG. 1B depicts how the FBGs 114 reflect the light pulse, according to another embodiment in which the optical fiber 112 comprises a third FBG 114 c. In FIG. 1B, the second FBG 114 b is equidistant from each of the first and third FBGs 114 a,c when the fiber 112 is not strained. The light pulse is propagating along the fiber 112 and encounters three different FBGs 114, with each of the FBGs 114 reflecting a portion 115 of the pulse back towards the interrogator 106. In embodiments comprising three or more FBGs 114, the portions of the sensing and reference pulses not reflected by the first and second FBGs 114 a,b can reflect off the third FBG 114 c and any subsequent FBGs 114, resulting in interferometry that can be used to detect an event along the fiber 112 occurring further from the optical source 101 than the second FBG 114 b. For example, in the embodiment of FIG. 1B, a portion of the sensing pulse not reflected by the first and second FBGs 114 a,b can reflect off the third FBG 114 c and a portion of the reference pulse not reflected by the first FBG 114 a can reflect off the second FBG 114 b, and these reflected pulses can interfere with each other at the interrogator 106.

Any changes to the optical path length of the sensor 116 result in a corresponding phase difference between the reflected reference and sensing pulses at the interrogator 106. Since the two reflected pulses are received as one combined interference pulse, the phase difference between them is embedded in the combined signal. This phase information can be extracted using proper signal processing techniques, such as phase demodulation. The relationship between the optical path of the sensor 116 and that phase difference (θ) is

${\theta = \frac{2\pi\;{nL}}{\lambda}},$ where n is the index of refraction of the optical fiber; L is the optical path length of the sensor 116; and λ is the wavelength of the optical pulses. A change in nL is caused by the fiber experiencing longitudinal strain induced by energy being transferred into the fiber. The source of this energy may be, for example, an object outside of the fiber experiencing the acoustics.

One conventional way of determining ΔnL is by using what is broadly referred to as distributed acoustic sensing (“DAS”). DAS involves laying the fiber 112 through or near a region of interest and then sending a coherent laser pulse along the fiber 112. As shown in FIG. 1C, the laser pulse interacts with impurities 113 in the fiber 112, which results in scattered laser light 117 because of Rayleigh scattering. Vibration or acoustics emanating from the region of interest results in a certain length of the fiber becoming strained, and the optical path change along that length varies directly with the magnitude of that strain. Some of the scattered laser light 117 is back scattered along the fiber 112 and is directed towards the optical receiver 103, and depending on the amount of time required for the scattered light 117 to reach the receiver and the phase of the scattered light 117 as determined at the receiver, the location and magnitude of the vibration or acoustics can be estimated with respect to time. DAS relies on interferometry using the reflected light to estimate the strain the fiber experiences. The amount of light that is reflected is relatively low because it is a subset of the scattered light 117. Consequently, and as evidenced by comparing FIGS. 1B and 1C, Rayleigh scattering transmits less light back towards the optical receiver 103 than using the FBGs 114.

DAS accordingly uses Rayleigh scattering to estimate the magnitude, with respect to time, of the event experienced by the fiber during an interrogation time window, which is a proxy for the magnitude of the event, such as vibration or acoustics emanating from the region of interest. In contrast, the embodiments described herein measure events experienced by the fiber 112 using interferometry resulting from laser light reflected by FBGs 114 that are added to the fiber 112 and that are designed to reflect significantly more of the light than is reflected as a result of Rayleigh scattering. This contrasts with an alternative use of FBGs 114 in which the center wavelengths of the FBGs 114 are monitored to detect any changes that may result to it in response to strain. In the depicted embodiments, groups of the FBGs 114 are located along the fiber 112. A typical FBG can have a reflectivity rating of 2% or 5%. The use of FBG-based interferometry to measure interference causing events offers several advantages over DAS, in terms of optical performance.

FIGS. 2-10 depict embodiments of methods, systems, and techniques for determining whether an acoustic event has occurred along a fluid conduit, such as a wellbore (e.g., well casing, production tubing) or pipeline. In certain embodiments, the system 100 of FIG. 1A obtains a measured acoustic signal using the sensors 116 placed along a pipeline to estimate the acoustic response of the path along which the acoustic signal propagates (hereinafter interchangeably referred to as the “acoustic path response”), which comprises the response of the fluid conduit, and the frequency content of external signals affecting the pipeline, which are modeled as acoustic source transfer functions that transform white noise acoustic sources. Being able to distinguish between changes in the acoustic path response and changes in the frequency content of the external signals affecting the pipeline may be used in leak detection and pipeline monitoring systems.

Technical challenges when developing a leak detection system comprise:

-   1. enabling real-time reporting of leaks; -   2. the ability to sense small leaks; -   3. automatically detecting leaks irrespective of environmental and     operating conditions; -   4. accurately estimating leak location; and -   5. avoiding false alarms, which may comprise identifying and     categorizing events other than leaks.

Certain embodiments described herein are able to continuously monitor pipelines using acoustic sensing equipment. FIG. 2 shows an example system 200 comprising a fluid conduit in the form of a pipeline 204 laid alongside a fiber conduit 202 within which is the optical fiber 112. A pair of acoustic events 208 a,b (generally, “acoustic events 208”) are depicted. The acoustic event 208 b on the pipeline 204 may represent, for example, a leak. As discussed above in respect of FIGS. 1A-1C, the FBGs 114 are sensitive to acoustics of various frequencies. The FBGs 114 accordingly comprise the functionality of a microphone and accelerometer. The conduit 202 is placed on or sufficiently near the pipeline 204 so as to be able to measure acoustics generated by the acoustic events 208. In certain example embodiments, the conduit 202 contacts the pipeline 204 or is within 10 cm, 20 cm, 30 cm, 40 cm, 50 cm, 60 cm, 70 cm, 80 cm, 90 cm, 1 m, 2 m, 3 m, 4 m, or 5 m of the pipeline 204. The FBGs 114 in the depicted embodiment are etched into the fiber 112 at 25 m intervals. Three sensors 116 a-c are accordingly depicted in FIG. 2, although in different embodiments (not depicted) there may be as few as two of the sensors 116 or many more than three of the sensors 116.

Each of the sensors 116 a-c in the depicted embodiment overlaps with a longitudinal segment of the pipeline 204, with none of the longitudinal segments overlapping each other and all of the longitudinal segments collectively forming a continuous portion of the pipeline 204. In different embodiments (not depicted), the longitudinal segments of the pipeline 204 that are monitored may not be continuous. For example, any two or more neighbouring longitudinal segments may be spaced apart so long as the neighbouring segments remain acoustically coupled to each other. Additionally or alternatively, in different embodiments (not depicted) the fiber 112 may not extend parallel with the pipeline 204. For example, in one example the fiber 112 is wound around segments of the pipeline 204 to increase sensitivity.

The system 200 of FIG. 2 permits continuous measurements to be obtained using the FBGs 114, thus facilitating real-time reporting of leaks. As different sensors correspond to different longitudinal segments of the pipeline 204, event localization becomes easier. Also, using the conduit 202, which may be plastic, to house the optical fiber 112 permits relatively straightforward installation. As discussed in more detail below, certain embodiments described herein are able to sense relatively small leaks and leaks occurring under low pipeline pressure or slack conditions.

Many conventional event detection systems are able to detect events 208, such as leaks or flow rate changes, when they have a priori knowledge about when the event is expected to occur. A more technically challenging problem is performing event detection without that a priori information. Similarly, many conventional event detection systems are able to detect events 208 during periods of relatively constant environmental or ambient conditions. A more technically challenging problem is performing event detection when one or both of operating and environmental conditions are changing.

At least some of the embodiments described herein address these technical challenges. The processor 102 extracts leak relevant features from the measured acoustic signal. Fluid escaping from the pipeline 204 may do any one or more of:

-   1. emit a broadband sound (a hiss); -   2. cause a vibration along the pipeline 204; -   3. cause a strain on the conduit 202 (as fluid escaping the pipeline     204 hits the conduit 202); -   4. decrease pressure in the pipeline 204; and -   5. related to any pressure decrease, cause a decrease in mass flow     rate in the pipeline 204 downstream of the leak.

Whenever a leak is present, a hole or crack in the pipeline 204 is also present. The leak itself may have different causes including any one or more of:

-   1. denting or buckling in the pipeline 204; -   2. a faulty seal between two flanges comprising the pipeline 204     (e.g., if the flanges are not bolted sufficiently tightly together); -   3. corrosion in the pipeline 204; -   4. movement of the ground surrounding the pipeline 204; and -   5. an intrusion attempt or accidental damage of the pipeline 204     using machinery.

The processor 102 distinguishes the aforementioned causes of the leak from normal or non-critical events affecting the pipeline 204, such as:

-   1. changes in fluid flow rate; -   2. changes in fluid density; -   3. external environmental sounds due to traffic, rivers, wind, rain,     etc.; -   4. changes in soil composition due to rain; -   5. changes in the pipeline 204, FBGs 114, or material surrounding     the pipeline 204 due to daily temperature cycles; -   6. vibrations due to machinery such as pumps and compressors     attached to or near the pipeline 204; and -   7. sensor errors and temporary sensor failures, etc.

Described herein is an approach to estimate both the acoustic path response, which in certain embodiments comprises the pipeline's 204 frequency response, and the frequency content of acoustic sources affecting the pipeline 204. By obtaining estimates of (and monitoring) both the pipeline's 204 frequency response and the acoustic sources' frequency content the processor 102 determines at least some of the features and causes of leaks listed above. For example:

-   1. A dent or buckling of the pipeline 204 changes the frequency     response of the longitudinal segment of the pipeline 204 comprising     that dent or buckling. -   2. Changing the pressure of the fluid in the pipeline 204 causes     changes in both the acoustic path response and the frequency content     of an acoustic source. The change in the acoustic path response does     not result from a change in the response of the pipeline 204 per se,     but the pressure of the fluid flowing through the pipeline 204.     Thus, by monitoring for these changes the processor 102 in certain     embodiments estimates the fluid pressure for each of the pipeline's     204 longitudinal segments. Once an estimate of the pressure for each     of the segments is obtained, in certain embodiments the processor     102 detects leaks by monitoring for drops in pressure along     downstream segments. -   3. If the frequency content of an acoustic source affecting a     particular longitudinal segment suddenly exhibits an increase in     broadband content, this may be due to the “hiss” of a leak in that     segment.

The processor 102, by being sensitive to several features of a leak, increases sensitivity to leaks and reduces the likelihood of a false positive occurring. The more features that are detected that are consistent with a leak, the more confidence associated with the processor's 102 determination that a leak is present.

The following assumptions apply to the pipeline 204 and system 200 of FIG. 2:

-   1. An event 208 acts as an acoustic source. Acoustic sources may     also comprise, for example, environmental noise or sound emitted by     a leak. -   2. An acoustic source “is attributed to” one of the sensors 116 when     the acoustics that that source emits are first detected by that one     of the sensors 116. In an embodiment in which the pipeline 204     extends substantially parallel to the ground, an acoustic source     accordingly is attributed to one of the sensors 116 when a line from     that acoustic source extending to the longitudinal segment of the     pipeline 204 monitored by that one of the sensors 116 is     perpendicular to that pipeline 204 segment. As discussed in further     detail below, all acoustic sources, whether they comprise events 208     or other acoustic generators, such as environmental noise or sound     emitted by a leak, attributed to one of the sensors 116 are summed     into a single acoustic source for that one of the sensors 116. -   3. The acoustic sources occur in, on, or near the pipeline 204. An     acoustic source is “near” a pipeline when the acoustics emitted by     the source are measurable by at least one of the sensors 116. -   4. Acoustic sources are mutually uncorrelated. -   5. Acoustic waves travel along an acoustic path that extends through     various media including the fluid in the pipeline 204, the pipeline     204 wall, and material surrounding the pipeline 204. -   6. Acoustic waves are reflected by valves, imperfections, etc. in     the pipeline 204, and interfaces in the material surrounding the     pipeline 204. -   7. Leaks are not always present, but when they occur they resemble a     broadband stochastic process.

A measured acoustic signal is a measurement of an acoustic signal resulting from a superposition of signals from multiple acoustic sources (each a “source signal”) that reach the sensor 116 via multiple paths; those acoustic sources may represent acoustic events 208, other sources, or both. Thus when an acoustic event 208 occurs along the pipeline 204, the processor 104 detects the event 208 using several of the nearest sensors 116 as the source signal generated by the event 208 propagates through the ground, pipeline 204 wall, and fluid inside the pipeline 204. Consequently, even though an event 208 is only attributed to one of the sensors 116, many of the sensors 116 are able to measure the event 208. Two features that distinguish a measured acoustic signal from the source signals that cause it are:

-   1. a single source signal generated by a single acoustic source near     the pipeline 204 is present in many of the measured acoustic signals     measured along different sensors 116; and -   2. a measured acoustic signal may separately comprise a source     signal and its reflection, which is treated as another source     signal. A source signal per se excludes its reflections.     As source signals travel through a medium to reach one or more of     the sensors 112 (possibly along many different paths), they are     affected by the medium through which they are travelling. Thus the     measured acoustic signal is a sum of filtered versions of one or     more source signals emanating from one or more acoustic sources. For     any given one of the sensors 116, the transfer function describing     the filtering of the source signal generated by the acoustic source     as it propagates to that one of the sensors 116 is called the “path     response” and in embodiments in which the pipeline 204 is being     monitored for leaks comprises the acoustic response of the     longitudinal segment of the pipeline 204 corresponding to that one     of the sensors 116.     Acoustics Propagation Model

FIG. 3 depicts a block diagram of a model 300 for acoustic wave propagation along the pipeline 204 in which the pipeline 204 is deemed to be extending in the left and right directions for convenience. The model 300 is not “identifiable” in that given proper data, estimates of all the desired transfer functions used in the model 300 cannot be determined. In FIG. 3 the model's 300 nodes and blocks are defined as follows:

-   1. w_(i) ^(λ) denotes an acoustic wave at sensor 116 i propagating     to the left; -   2. w_(i) ^(r) denotes an acoustic wave at sensor 116 i propagating     to the right; -   3. G₁₂ ^(i) denotes the path response of an acoustic wave     propagating to the left from sensor 116 i+1 to i; -   4. G₂₁ ^(i) denotes the path response of an acoustic wave     propagating to the right from sensor 116 i to i+1; -   5. G₁₁ ^(i) denotes the path response of an acoustic wave that was     traveling to the right at sensor 116 i, and was reflected (i.e. is     now traveling to the left) before it reached sensor 116 i+1; -   6. G₂₂ ^(i) denotes the path response of an acoustic wave that was     traveling to the left at sensor i+1 and was reflected before it     reached sensor 116 i; -   7. e_(i) denotes an acoustic source that is attributed to in sensor     116 i. Sources are represented as white stochastic processes (white     noise) and are hereinafter interchangeably referred to as “external     signals” e_(i); -   8. H_(r) ^(i) denotes the frequency content of the source signal     originating from source e_(i) traveling to the right. It is assumed     that the source signal generated by source i predominantly follows     the path of the other acoustic waves traveling to the right; and -   9. H₈₀ ^(i) denotes the frequency content of the source signal     originating from source e_(i) traveling to the left. It is assumed     that the source signal generated by source i predominantly follows     the path of the other acoustic waves traveling to the left.     In FIG. 3, the acoustic path response for one of the sensors 116 i     is characterized by G₁₂ ^(i), G₂₁ ^(i), G₁₁ ^(i), and G₂₂ ^(i).

An acoustic measurement at sensor 116 i at time t is modeled as: w _(i)(t)=F _(i)(q)(w_(i) ^(r)(t)+

(t))+s _(i)(t)   (1) where F_(i) is the acoustic sensor frequency response, and s_(i) is sensor noise (i.e. measurement error). The sensor 116 measures acoustic waves traveling in both directions. Unless otherwise stated herein, s_(i) is assumed to be very small compared to e_(i) and accordingly can for practical purposes be dropped from the equations. A component of the sensor frequency response is an integration over the sensor's 116 length.

The transfer functions G₁₂ ^(i), G₂₁ ^(i), G₁₁ ^(i), and G₂₂ ^(i) describe the acoustic path response; that is, the acoustic response of the path the acoustic wave travels, which in the depicted embodiment comprises the pipeline 204. Thus these transfer functions are affected by physical changes in the pipeline 204 due to dents, corrosion, fluid density, fluid flow rate, fluid pressure within the pipeline 204, material surrounding the pipeline 204, and the like. On the other hand, the transfer functions H_(r) ^(i) and H_(λ) ^(i) describe the filter that shapes the source signals affecting the pipeline 204 as generated by the external sources e_(i). As discussed above, those acoustic waves are by definition white noise, and so the filter H_(r) ^(i) changes according to the frequency content of the external sources e_(i) affecting the pipeline 204 such as wind, machinery, traffic noise, river noise, etc.

Given the measurements w_(i), i=1,2,K the transfer functions G₁₂ ^(i), G₂₁ ^(i), G₁₁ ^(i), G₂₂ ^(i), H_(r) ^(i), and H₈₀ ^(i) i=1,2,K in the model 300 shown in FIG. 3 are not identifiable primarily due to the fact that the measured acoustic signal is a superposition of acoustic waves (filtered source signals) travelling in all directions.

The mathematical relationship between the measured variables w_(i), i=1,2,K is determined below. A mathematical representation of the equations illustrated in FIG. 3 for a six sensor setup is:

$\begin{matrix} {\quad{\begin{bmatrix} \omega_{1}^{\ell} \\ \omega_{1}^{r} \\ \omega_{2}^{\ell} \\ \omega_{2}^{r} \\ \omega_{3}^{\ell} \\ \omega_{3}^{r} \\ \omega_{4}^{\ell} \\ \omega_{4}^{r} \\ \omega_{5}^{\ell} \\ \omega_{5}^{r} \\ \omega_{6}^{\ell} \\ \omega_{6}^{r} \end{bmatrix} = {\quad{\quad{\left\lbrack \begin{matrix} \; & G_{11}^{1} & G_{12}^{1} & \; & \; & \; & \; & \; & \; & \; & \; & \; \\ G_{22}^{0} & \; & \; & \; & \; & \; & \; & \; & \; & \; & \; & \; \\ \; & \; & \; & G_{11}^{2} & G_{12}^{2} & \; & \; & \; & \; & \; & \; & \; \\ \; & G_{21}^{1} & G_{22}^{1} & \; & \; & \; & \; & \; & \; & \; & \; & \; \\ \; & \; & \; & \; & \; & G_{11}^{3} & G_{12}^{3} & \; & \; & \; & \; & \; \\ \; & \; & \; & G_{21}^{2} & G_{22}^{2} & \; & \; & \; & \; & \; & \; & \; \\ \; & \; & \; & \; & \; & \; & \; & G_{11}^{4} & G_{11}^{4} & \; & \; & \; \\ \; & \; & \; & \; & \; & G_{21}^{3} & G_{21}^{3} & \; & \; & \; & \; & \; \\ \; & \; & \; & \; & \; & \; & \; & \; & \; & G_{11}^{5} & G_{12}^{5} & \; \\ \; & \; & \; & \; & \; & \; & \; & G_{21}^{4} & G_{22}^{4} & \; & \; & \; \\ \; & \; & \; & \; & \; & \; & \; & \; & \; & \; & \; & G_{11}^{6} \\ \; & \; & \; & \; & \; & \; & \; & \; & \; & G_{21}^{5} & G_{22}^{5} & \; \end{matrix} \right\rbrack{\quad{\begin{bmatrix} \omega_{1}^{\ell} \\ \omega_{1}^{r} \\ \omega_{2}^{\ell} \\ \omega_{2}^{r} \\ \omega_{3}^{\ell} \\ \omega_{3}^{r} \\ \omega_{4}^{\ell} \\ \omega_{4}^{r} \\ \omega_{5}^{\ell} \\ \omega_{5}^{r} \\ \omega_{6}^{\ell} \\ \omega_{6}^{r} \end{bmatrix} + {{\begin{bmatrix} \; & H_{\lambda}^{1} & \; & \; & \; & \; & \; & \; \\ H_{r}^{0} & H_{r}^{1} & \; & \; & \; & \; & \; & \; \\ \; & \; & H_{\lambda}^{2} & \; & \; & \; & \; & \; \\ \; & \; & H_{r}^{2} & \; & \; & \; & \; & \; \\ \; & \; & \; & H_{\lambda}^{3} & \; & \; & \; & \; \\ \; & \; & \; & H_{r}^{3} & \; & \; & \; & \; \\ \; & \; & \; & \; & H_{\lambda}^{4} & \; & \; & \; \\ \; & \; & \; & \; & H_{r}^{4} & \; & \; & \; \\ \; & \; & \; & \; & \; & H_{\lambda}^{5} & \; & \; \\ \; & \; & \; & \; & \; & H_{r}^{5} & \; & \; \\ \; & \; & \; & \; & \; & \; & H_{\lambda}^{6} & H_{\lambda}^{7} \\ \; & \; & \; & \; & \; & \; & H_{r}^{6} & \; \end{bmatrix}\begin{bmatrix} e_{0} \\ \begin{matrix} e_{1} \\ e_{2} \\ e_{3} \\ e_{4} \\ e_{5} \\ e_{6} \\ e_{7} \end{matrix} \end{bmatrix}}.}}}}}}}} & (2) \end{matrix}$

Equation (2) can be expressed as: w ^(m)(t)=G ^(m)(q)w ^(m)(t)+H ^(m)(q)e ^(m)(t)   (3)

An equation in terms of w_(i)'s as defined in Equation (1) is desirable. The expression for w^(m) in terms of only e^(m) is w ^(m)=(I−G ^(m))⁻¹ H ^(m) e ^(m)   (4) where the inverse is guaranteed to exist because I−G^(m) is monic. In order to obtain an expression with a vector of F_(i)(q)(w_(i) ^(r)+w_(i) ^(λ)), i=1,2,K, on the left hand side, premultiply Equation (4) by

$M = \begin{bmatrix} F_{1} & F_{1} & \; & \; & \; & \; & \; & \; & \; & \; & \; & \; \\ \; & \; & F_{2} & F_{2} & \; & \; & \; & \; & \; & \; & \; & \; \\ \; & \; & \; & \; & F_{3} & F_{3} & \; & \; & \; & \; & \; & \; \\ \; & \; & \; & \; & \; & \; & F_{4} & F_{4} & \; & \; & \; & \; \\ \; & \; & \; & \; & \; & \; & \; & \; & F_{5} & F_{5} & \; & \; \\ \; & \; & \; & \; & \; & \; & \; & \; & \; & \; & F_{6} & F_{6} \end{bmatrix}$ resulting in w(t)=M(q)(I−G ^(m)(q))⁻¹ H ^(m)(q)e(t)=W(q)e(t)   (5) where the elements of w are w_(i) as defined in Equation (1) and w(q)=M(q)(I−G^(m)(q))⁻¹H^(m)(q). Two points about Equation (5) are:

-   1. The matrix W is a full matrix (i.e. all entries are non-zero). In     particular, each entry is a product of H_(r) ^(i), H_(λ) ^(i), and     G_(mn) ^(i), m,n=1,2, and i=1,2,3,K. -   2. Due to the structure of the network shown in FIG. 3, W can be     factored into two matrices of transfer functions, where one of the     matrices of transfer functions depends only on the acoustic path     responses G₁₁ ^(i), G₁₂ ^(i), G₂₁ ^(i), i=1,2,K.

Determining the acoustic path responses the pipeline 204 segments being monitored by the sensors 116 is desired. Because each element in W is a function of G₁₁ ^(i), G₁₂ ^(i), G₂₁ ^(i), G₂₂ ^(i), H_(r) ^(i)'s and H₈₀ ^(i), i=1,2,K it is not sufficient to monitor the transfer functions of W. In order to independently monitor the acoustic path responses from the acoustic sources e, affecting the pipeline, W is factored. W can be factored as: W(q)=F(q) (I−G(q))⁻¹ H(q)   (6) where F=diag(F₁,K,F₆), and

$\;{{G = \begin{bmatrix} 0 & G_{12} & \; & \; & \; & \; \\ G_{21} & 0 & G_{23} & \; & \; & \; \\ \; & G_{32} & 0 & G_{34} & \; & \; \\ \; & \; & G_{43} & 0 & G_{45} & \; \\ \; & \; & \; & G_{54} & 0 & G_{56} \\ \; & \; & \; & \; & G_{65} & 0 \end{bmatrix}},{H = \begin{bmatrix} H_{10} & H_{1} & H_{12} & \; & \; & \; & \; & \; \\ \; & H_{21} & H_{2} & H_{23} & \; & \; & \; & \; \\ \; & \; & H_{32} & H_{3} & H_{34} & \; & \; & \; \\ \; & \; & \; & H_{43} & H_{4} & H_{45} & \; & \; \\ \; & \; & \; & \; & H_{54} & H_{5} & H_{56} & \; \\ \; & \; & \; & \; & \; & H_{65} & H_{6} & H_{67} \end{bmatrix}},}$ where

${G_{ij} = {{\frac{G_{12}^{i}N_{i - 1}}{D_{{i - 1},i}}\mspace{14mu}{if}\mspace{11mu} i} < j}},{G_{ij} = {{\frac{G_{21}^{j}N_{i}}{D_{{i - 1},i}}\mspace{14mu}{if}\mspace{11mu} i} < j}},{H_{i} = \frac{{{H_{\ell}^{i}\left( {1 + G_{22}^{i}} \right)}N_{i - 1}} + {{H_{r}^{i}\left( {1 + G_{11}^{i - 1}} \right)}N_{i}}}{D_{{i - 1},i}}},{H_{ij} = {{- G_{ij}}H_{p}^{i}}},{{{if}\mspace{11mu} i} < j},{H_{ij} = {{- G_{ij}}H_{\ell}^{i}}},{{{if}\mspace{11mu} i} < j},$ where

${N_{k} = {\det\begin{bmatrix} {1 + G_{11}^{k}} & G_{12}^{k} \\ G_{21}^{k} & {1 + G_{22}^{k}} \end{bmatrix}}},{D_{mn} = {{\det\begin{bmatrix} {1 + G_{11}^{m}} & G_{12}^{m} & \; & \; \\ \; & 1 & G_{11}^{n} & G_{12}^{n} \\ G_{21}^{m} & G_{22}^{m} & 1 & \; \\ \; & \; & G_{21}^{n} & {1 + G_{22}^{n}} \end{bmatrix}}.}}$

Using the factorization of Equation (6), a network equation relating the measured variables is: w(t)=W(q)e(t) F ⁻¹(q)w(t)=G(q)F ⁻¹(q)w(t)+H(q)e(t) w(t)=F(q)G(q)F ⁻¹(q)w(t)+F(q)H(q)e(t),   (7) where G, H, and F are defined in Equation (6).

Two points about Equation (7) are:

-   1. G is only a function of G₁₁ ^(i), G₁₂ ^(i), G₂₁ ^(i), G₂₂ ^(i),     i=1,2,K. -   2. H is not square.

The first point means that the dynamics of the acoustic path (represented by the acoustic path responses G₁₁ ^(i), G₁₂ ^(i), G₂₁ ^(i), and G₂₂ ^(i), i=1,2,K) can be identified independently from the external signals' e_(i) frequency content (represented by H_(λ) ^(i) and H_(r) ^(i), i=1,2,K).

The second point is an issue in that rectangular noise models may not be identifiable. In the following text a noise model that is statistically equivalent to H in Equation (7) is derived, but it is square. Two statistically equivalent noise models H₁ and H₂ are such that the statistics of ν₁ and ν₂ are the same for both noise models (where ν_(i)=H_(i)e, i=1,2, where e_(i) is a white noise process). In particular ν₁ and ν₂ are statistically equivalent if they have the same power spectral density Φ_(v) _(i) =H(e^(jω))H(e^(−jω))σ_(e) ₁ ², where σ_(e) _(i) ² is the power of the white noise process e_(i)(t).

Noise models are closely related to spectral factors. By the spectral factorization theorem, any power spectral density matrix Φ(z) can be uniquely factored as Φ(z)=H(z)H(z⁻¹)^(T) where H(z) is a (square) monic stable, minimum phase transfer matrix. For Equation (7) the power spectral density matrix of the noise is equal to:

$\begin{matrix} {{\Phi_{v}(z)} = {{{H(z)}{H\left( z^{- 1} \right)}^{T}} = \begin{bmatrix} {A_{11}(z)} & {B_{12}(z)} & {C_{13}(z)} & 0 & 0 & 0 \\ {B_{21}(z)} & {A_{22}(z)} & {B_{23}(z)} & {C_{24}(z)} & 0 & 0 \\ {C_{31}(z)} & {B_{32}(z)} & {A_{33}(z)} & {B_{34}(z)} & {C_{35}(z)} & 0 \\ 0 & {C_{42}(z)} & {B_{43}(z)} & {A_{44}(z)} & {B_{45}(z)} & {C_{46}(z)} \\ 0 & 0 & {C_{53}(z)} & {B_{54}(z)} & {A_{55}(z)} & {B_{56}(z)} \\ 0 & 0 & 0 & {C_{64}(z)} & {B_{65}(z)} & {A_{66}(z)} \end{bmatrix}}} & (8) \end{matrix}$ where A _(ii)(z)=H _(i,i−1)(z)H _(i,i−1)(z ⁻¹)+H _(ii)(z)H _(ii)(z ⁻¹)+H _(i,i+1)(z)H _(i,i+1)(z ⁻¹) B _(ij)(z)=H _(ij)(z)H _(jj)(z ⁻¹)+H _(ii)(z)H _(ji)(z ⁻¹) C _(ij)(z)=H _(i,i−1)(z)H _(j,j−1)(z ⁻¹).

Note that the power spectral density in Equation (8) is 5-diagonal para-Hermitian matrix. Para-Hermitian means that the (i, j) th entry, Φ_(ij)(z)=Φ_(ji)(z⁻¹). Moreover, no entries in the diagonal bands are zero, as long as there is no situation where C_(ij) or B_(ij) are equal to zero. From Equations (7) and (8):

$C_{ij} = {{{H_{i,{i - 1}}(z)}{H_{j,{j - 1}}\left( z^{- 1} \right)}} = {\frac{{G_{12}^{i - 1}(z)}{N_{i - 1}(z)}{H_{r}^{i - 1}(z)}{G_{21}^{i - 1}\left( z^{- 1} \right)}{N_{j}\left( z^{- 1} \right)}{H_{\lambda}^{j + 1}\left( z^{- 1} \right)}}{{D_{{i - 1},{i - 1}}(z)}{D_{{j - 1},{j - 1}}\left( z^{- 1} \right)}}.}}$

It follows that elements C_(ij) only equal zero if either G₁₂ ^(i−1) or G₂₁ ^(i−1) are zero, which means there is no acoustic energy transfer between the sensors 116. This, in practice, is unlikely. The same argument can be made for the elements B_(ij). A 5-diagonal matrix where none of the elements in the diagonal bands are zero is hereinafter referred to as a full 5-diagonal matrix. The following lemma shows that the spectral factor of a full 5-diagnal matrix is nearly a full 3-diagonal matrix.

Lemma 1: Let Φ_(ν) be an n×n Hermitian matrix. Let H be the unique, monic, stable and minimum phase spectral factor of Φ_(ν). If Φ is a full 5-diagonal matrix then H is a full 3-diagonal matrix with possibly non-zero entries in the (3,1) and (n−2,n) positions and possibly zero entries in the (2,1) and (n−1,n) positions.

From Equation (8) and Lemma 1 it follows that ν=He can be equivalently modelled as ν=

where

is a square, monic, stable, minimum phase full 3-diagonal matrix. Thus, H can be replaced by

in Equation (7) without any changes to w. Consequently, the final model for the acoustic sensor setup is: w(t)=F(q)G(q)F ⁻¹(q)w(t)+F(q)H̆(q)ĕ(t).   (9)

A graphical representation of Equation (9) is shown as a model 400 in FIG. 4. The model 400 depicts measured variables w_(i), i=1,K,6, and external sources e_(i), i=1,K,6. The relationship between measurements w_(i), i=1,2,K and sources

, i=1,2,K can be determined from Equation (9) as

$\begin{matrix} {{w(t)} = {\left( {I - {{F(q)}{G(q)}{F^{- 1}(q)}}} \right)^{- 1}{F(q)}{\overset{\Cup}{H}(q)}{\overset{\Cup}{e}(t)}}} & (10) \\ {{= {{F(q)}\left( {I - {G(q)}} \right)^{- 1}{\overset{\Cup}{H}(q)}{\overset{\Cup}{e}(t)}}}{{{Let}\mspace{14mu}{\overset{\Cup}{W}(q)}} = {{F(q)}\left( {I - {G(q)}} \right)^{- 1}{{\overset{\Cup}{H}(q)}.}}}} & (11) \end{matrix}$

Certain points about Equation (9) are summarized in the following list:

-   1. The transfer functions G_(ij), i, j=1,2,K are functions of only     the acoustic path responses, i.e. only G₁₁ ^(i), G₁₂ ^(i), G₂₁ ^(i),     and G₂₂ ^(i), i=1,2,K as defined in Equation (2). Thus a change in     the acoustic path response is reflected by a change in one or more     G_(ij), i, j=1,2,K. In contrast, a change in the loudness or     frequency content of the acoustic sources (external signals e_(i))     does not change any G_(ij), i, j=1,2,K. -   2. A change in the frequency content of the external signals e_(i)     affecting the pipeline 204 results in a change in the acoustic     source transfer functions H_(ij), i, j=1,2,K. -   3. Recall that F is a diagonal matrix of the sensor response     functions. If each sensor has approximately the same response then     F(q)G(q)F⁻¹(q) is approximately independent of the sensor response.     The dominant feature of the sensor response is due to the fact that     each of the sensors 116 is distributed.

Using the first two points it is possible to distinguish between changes in the acoustic path response and changes in the frequency content of the external signals e_(i) affecting the pipeline 204.

Implementation

The methods and techniques described above may be implemented using, for example, Matlab™ software. The method to obtain estimates of F(q)G(q)F⁻¹(q) and F(q)

(q) in Equation (9) is split into three actions. In the first action the processor 102 estimates the matrix

in Equation (10) from the data. In the second action the processor 102 factors the estimated

into F(q)G(q)F⁻¹(q) and F(q)

(q) as defined in Equation (9). In the last action the processor 102 further refines the estimates of F(q)G(q)F⁻¹(q) and F(q)

(q) to reduce prediction error.

The method for the first action, i.e. estimating

in Equation (9) from data, is a by-product of estimating the source powers using, for example, a technique such as that presented in chapter 6 of Huang, Y., Benesty, J., and Chen, J. (2006), Acoustic MIMO Signal Processing, Signals and Communication Technology, Springer-Verlag Berlin Heidelberg and in chapter 7 of Liung, L. (1999), System Identification, Theory for the User, 2^(nd) Edition, Prentice Hall, the entireties of both of which are hereby incorporated by reference. In this action, the processor 102 determines an estimate of

, where each element of

(q, θ) is a parameterized transfer function that is parameterized using a Finite Impulse Response (FIR) structure, i.e. the elements are parameterized as: W _(ij)(q,θ)=θ_(ij) ⁽¹⁾ q ^(−d) ^(ij) +θ_(ij) ⁽²⁾ q ^(−d) ^(ij) ⁻¹+Λ+θ_(ij) ^((m)) q ^(−d) ^(ij) ^(−m) ,i,j=1,2,K,i≠j, W _(ii)(q,θ)=1+θ_(ii) ⁽¹⁾ q ⁻¹+Λ+θ_(ii) ^((m)) q ^(−m) ,i=1,2,K, where d_(ij) is the delay is the delay of the (i,j)th off-diagonal transfer function representing the time it takes for an acoustic wave to travel between the sensors 116 and θ_(ij) is a parameter to be estimated.

When performing the second action, the processor 102 factors the estimate

(q,{circumflex over (θ)}) into G and H, where {circumflex over (θ)} is an estimated version of θ. The processor 102 in one example embodiment does this factorization using a linear regression. It is desirable to factor

as: W̆(q,θ)=B ⁻¹(q,β)A(q,α),   (12) where α and β are parameter vectors that define A and B. From Equation (9), A(q, α) is an estimate of F(q)

(q), and B(q, β) is an estimate of F(q)(I−G(q))⁻¹F⁻¹(q). In addition, from Equation (9) the matrices F(q)

(q) and F(q)(I−G(q))⁻¹F⁻¹(q) have a particular structure. Therefore, A and B are parameterized with the same matrix structure:

${{A\left( {q,\alpha} \right)} = \begin{bmatrix} 1 & {A_{12}\left( {q,\alpha} \right)} & \; & \; & \; \\ {A_{21}\left( {q,\alpha} \right)} & 1 & {A_{23}\left( {q,\alpha} \right)} & \; & \; \\ \; & {A_{32}\left( {q,\alpha} \right)} & O & O & \; \\ \; & \; & O & 1 & {A_{{L - 1},L}\left( {q,\alpha} \right)} \\ \; & \; & \; & {A_{L,{L - 1}}\left( {q,\alpha} \right)} & 1 \end{bmatrix}},{{B\left( {q,\beta} \right)} = \begin{bmatrix} {B_{11}\left( {q,\beta} \right)} & {B_{12}\left( {q,\beta} \right)} & \; & \; & \; \\ {B_{21}\left( {q,\beta} \right)} & {B_{22}\left( {q,\beta} \right)} & {B_{23}\left( {q,\beta} \right)} & \; & \; \\ \; & {B_{32}\left( {q,\beta} \right)} & O & O & \; \\ \; & \; & O & {B_{{L - 1},{L - 1}}\left( {q,\beta} \right)} & {B_{{L - 1},L}\left( {q,\beta} \right)} \\ \; & \; & \; & {B_{L,{L - 1}}\left( {q,\beta} \right)} & {B_{LL}\left( {q,\beta} \right)} \end{bmatrix}},$ where each A_(ij)(q, α), and B_(ij)(q, β) are parameterized transfer functions. Each A_(ij)(q, α), and B_(ij)(q, β) are parameterized using a FIR structure, although in different embodiments (not depicted) a different parameterization may be used. This choice ensures uniqueness of the estimates and also makes the estimation of α and β easier. In particular the processor 102 parameterizes A_(ij)(q, α), and B_(ij)(q, β) as A _(ij)(q,α)=α_(ij) ⁽¹⁾ q ^(−d) ^(ij) +α_(ij) ⁽²⁾ q ^(−d) ^(ij) ⁻¹+Λ+α_(ij) ^((m)) q ^(−d) ^(ij) ^(−m) ,i,j=1,2,K, B _(ij)(q,β)=β_(ij) ⁽¹⁾ q ^(−d) ^(ij) +β_(ij) ⁽²⁾ q ^(−d) ^(ij) ⁻¹+Λ+β_(ij) ^((m)) q ^(−d) ^(ij) ^(−m) ,i,j=1,2,K,i≠j, B _(ii)(q,β)=1+β_(ii) ⁽¹⁾ q ⁻¹+Λ+β_(ii) ^((m)) q ^(−m) ,i=1,2,K.

The parameterization is entirely defined by α, β, d_(ij), i, j=1,2,K, and m.

From Equation (12) it follows that B(q,β)W(q,{circumflex over (θ)})=A(q,α).   (13)

Because W, A, and B are parameterized using an FIR structure, α and β appear linearly in Equation (13). This means that the equations can be re-organized to gather all elements of α and β into a vector:

${{\begin{bmatrix} P & {M\left( \hat{\theta} \right)} \end{bmatrix}\begin{bmatrix} \alpha \\ \beta \end{bmatrix}} = {\zeta\left( \hat{\theta} \right)}},$ where ζ({circumflex over (θ)}) is a vector. Due to the structure of A and B because W and B are parameterized with monic transfer functions on the diagonal, it follows that [P M ({circumflex over (θ)})] is square and always full rank. Therefore, estimates of α and β can be obtained as:

$\begin{matrix} {\begin{bmatrix} \hat{\alpha} \\ \hat{\beta} \end{bmatrix} = {\begin{bmatrix} P & {M\left( \hat{\theta} \right)} \end{bmatrix}^{- 1}{{\zeta\left( \hat{\theta} \right)}.}}} & (14) \end{matrix}$

In certain embodiments the processor 102 uses any one or more of several methods to further refine {circumflex over (α)} and {circumflex over (β)} such that they better represent the data. For example, the processor 102 may use a Weighted Null Space Least Squares (WNLS) method. The processor 102 may use WNLS to iteratively minimize the prediction error by iteratively adjusting the value of {circumflex over (θ)}.

For example, in certain example embodiments the processor 102 iteratively selects values of {circumflex over (θ)} until the prediction error converges such that a stopping criterion is satisfied. In embodiments in which the processor 102 selects {circumflex over (θ)} using Equation (21), for example, the processor 102 may iteratively select {circumflex over (θ)} until the difference between successive iterations is small enough to satisfy the stopping criterion. In one specific example, the processor 102 ceases iterating when successive iterations of the slope of the objective function being minimized is small enough (e.g., a difference of less than 1×10⁻⁴) to satisfy the stopping criterion.

The processor 102 also determines when an estimated acoustic path response and/or an acoustic source transfer function has changed. In order to continuously monitor the pipeline 204, the processor 102 segments the data coming collected using the fiber 112 into blocks of a certain duration, each of which in the depicted embodiment is one minute long. For each block of data, the processor 102 determines estimates of F(q)G(q)F⁻¹(q) and F(q)

(q).

The result is that the processor 102 determines a sequence of estimated transfer functions in the form of the acoustic path responses and the acoustic source transfer functions. The processor 102 then monitors the estimated transfer functions for changes. Depending on which transfer function changes, the change may represent a change in the acoustic path (e.g., a hole in the pipeline 204) or a change in the frequency content of the external sources e, (e.g., a truck driving in the vicinity of the pipeline 204). Because the processor 102 compares two estimated transfer functions, in certain embodiments the processor 102 determines the confidence bounds for each transfer function. The processor 102 then uses the confidence bounds to determine the statistical distance between the two estimated frequency response functions at a particular frequency. The processor 102 does this as follows.

Let G(e^(jω), {circumflex over (θ)}) and

(e^(jω), {circumflex over (θ)}) denote the frequency response functions of the estimates of G and

. The covariance of the frequency response functions of the estimated transfer functions is

${{{Cov}\begin{bmatrix} {G\left( {e^{j\;\omega},\hat{\theta}} \right)} \\ {\overset{(}{H}\left( {e^{j\;\omega},\hat{\theta}} \right)} \end{bmatrix}} \approx {\frac{1}{N}{T\left( {e^{j\;\omega},\theta_{0}} \right)}P_{\theta}{T\left( {e^{{- j}\;\omega},\theta_{0}} \right)}}},$ where

${{T\left( {e^{j\;\omega},\theta} \right)} = \begin{bmatrix} {\frac{d}{d\;\theta}{G\left( {e^{j\;\omega},\theta} \right)}} & {\frac{d}{d\;\theta}{\overset{(}{H}\left( {e^{j\;\omega},\hat{\theta}} \right)}} \end{bmatrix}},$ and P_(θ) is the covariance matrix of the estimated parameter vector: P ₇₄ =(Ē[ψ(t,θ ₀)Λ₀ ⁻¹ψ^(T)(t,θ ₀)])⁻¹, where

${{\psi\left( {t,\theta} \right)} = {{- \frac{d}{d\;\theta}}{ɛ\left( {t,\theta} \right)}}},$ where ε is the prediction error.

Let the variance of G(e^(jω), {circumflex over (θ)}) and H(e^(jω), {circumflex over (θ)}) be denoted σ_(G) ²(e^(jω)) and σ_(H) ²(e^(jω)) respectively. Then the statistical difference between two estimates G(e^(jω), {circumflex over (θ)}₁) and G(e^(jω), {circumflex over (θ)}₂) is:

$\begin{matrix} {{d\left( e^{j\;\omega} \right)} = {\sqrt{2}\frac{{G\left( {e^{j\;\omega},{\hat{\theta}}_{1}} \right)} - {G\left( {e^{j\;\omega},{\hat{\theta}}_{2}} \right)}}{\sqrt{{\sigma_{G}^{2}\left( {e^{j\;\omega},\theta_{1}} \right)} - {\sigma_{G}^{2}\left( {e^{j\;\omega},\theta_{2}} \right)}}}}} & (15) \end{matrix}$

The processor 102 determines the statistical distance at each frequency of the frequency response functions. From Equation (15) it follows that if the estimates G(e^(jω), {circumflex over (θ)}₁) and G(e^(jω), {circumflex over (θ)}₂) are very different at frequencies where the variance of the estimates are small, then the statistical distance between them is large. In contrast, if the estimates G(e^(jω), {circumflex over (θ)}₁) and G(e^(jω), {circumflex over (θ)}₂) are very different at frequencies where the variance of the estimates is large, then the statistical distance between the estimates is not as big as before. Thus, by using statistical difference to monitor for changes in transfer functions, the processor 102 incorporates uncertainty associated with the estimates into the monitoring method.

Accordingly, in one embodiment consistent with the above description, the method for detecting whether the acoustic event has occurred comprises, given periodically refreshed data sets of length N obtained from L channels of the sensor as shown in FIG. 2:

-   1. Choose parameterization for the matrices W(q,θ), A(q,α) and     B(q,β). -   2. For each new data set that is received, the processor 102:     -   (a) Estimates         in Equation (9) by estimating source powers.     -   (b) Using         ({circumflex over (θ)}), determines estimates of F(q)G(q)F⁻¹(q)         and F(q)         (q), as outlined in Equations (12) to (14).     -   (c) Refines the estimates of F(q)G(q)F⁻¹(q) and F(q)         (q) using WNLS.     -   (d) Determines the variance of the frequency response functions         of the estimated transfer functions. -   3. Determine the statistical distance to the previous estimates     using Equation (15).

One example embodiment of this method is depicted in FIG. 10, which may be expressed as computer program code and performed by the processor 102. In FIG. 10, the processor 102 begins at block 1002 and proceeds to block 1004 where it determines a linear relationship between the measured acoustic signal and the white noise acoustic source (external source e_(i)) located along a longitudinal segment of the fluid conduit overlapping the sensor. The processor 102 then proceeds to block 1006 where, from the linear relationship, it determines an acoustic path response and an acoustic source transfer function that transforms the white noise acoustic source. In one embodiment the processor 102 does this by determining F(q)G(q)F⁻¹(q) and F(q)

(q) as described above. Determining F(q)G(q)F⁻¹(q) and F(q)

(q) for a portion of the fiber 112 results in determining the acoustic path response and acoustic source transfer function for each of the sensors 116 comprising that portion of the fiber 112. The processor 102 performs blocks 1004 and 1006 for all of the sensors 116.

The processor 102 then proceeds to block 1008 where it monitors over time variations in one or both of the acoustic path responses and acoustic source transfer functions. An example of this is determining statistical differences of one or both of the acoustic path responses and acoustic source transfer functions as described above.

The processor 102 subsequently proceeds to block 1010 where it determines whether at least one of the variations exceeds an event threshold. An example of this is determining whether the determined statistical differences exceed the event threshold.

If not, the processor 102 proceeds to block 1014 and the method of FIG. 10 ends.

If at least one of the power estimates exceeds the event threshold, the processor 102 proceeds from block 1010 to 1012. At block 1012, the processor 102 attributes the acoustic event 208 to one of the sensors 116 for which the acoustic path response or acoustic source transfer function varied in excess of the event threshold. For example, the processor 102 may attribute the acoustic event 208 to the one of the sensors 116 for which the acoustic path response or acoustic source transfer function most exceeds the event threshold. Alternatively, in embodiments in which there are multiple acoustic events, the processor 102 may attribute one of the acoustic events 208 to each of the sensors 116 for which the acoustic path response or acoustic source transfer function exceeds the event threshold. In one example embodiment in which there is only one acoustic event 208, the event threshold is selected such that the acoustic path response or acoustic source transfer function exceeds the event threshold for only one of the sensors 116, and the acoustic event 208 is attributed to that sensor 116.

In embodiments in which there are multiple acoustic events 208, the power estimates of the acoustic sources attributed to multiple of the sensors 116 may exceed the event threshold; in the current embodiment, the processor 102 attributes a different acoustic event 208 to each of the sensors 116 i to which is attributed an acoustic source that exceeds the event threshold. The event threshold for the sensors 116 may be identical in certain embodiments; in other embodiments, the event thresholds may differ for any two or more of the sensors 116.

In embodiments in which the acoustic event 208 is the leak, the processor 102 determines the acoustic event as affecting the longitudinal segment of the pipeline 204 corresponding to the sensor 116 to which the acoustic event is attributed.

In at least some example embodiments, a test signal may be generated and used to do one or both of generate and test the acoustic path responses and the acoustic source transfer functions. The test signal may, for example, be an impulse signal, or white noise comprising a wide acoustic band, a frequency sweep signal, or pings of various frequencies. Using a known acoustic input allows better estimation of the acoustic path response and the acoustic source transfer functions because it reduces the uncertainty regarding the input signals and improves the transfer function calculations based on the relationship between the measured output and the known input signals. In embodiments in which the input signal is or approximates an ideal impulse signal, the frequency domain conversion of the measured output signal is or approximates the acoustic path response.

EXAMPLES

FIG. 5 depicts test equipment used to validate the method described above, while FIGS. 6-8 depict the associated experimental results. More particularly, FIG. 5 depicts equipment 500 comprising a rectangular piece of acoustic foam 502 laid on the floor 503 of a room. Outside of the foam 502 and adjacent the centers of the foam's 502 short sides are two pieces of PVC pipe around which the optical fiber 112 and FBGs 114 are wrapped, and which consequently act as the sensors 116. A first speaker 504 a and a second speaker 504 b are adjacent the PVC pipe (the speakers 504 a,b are collectively “speakers”).

Two uncorrelated sequences of Gaussian noise were generated. Each signal was split into 4 parts. Parts 1-4 were filtered by a Chebyshev Type 1 Bandpass filter of order 2, 3, 4, and 5, respectively. The signals were played over the speakers 504 a,b. The ordering of the first signal was r₁, r₂, r₃, r₄, and r₁, where r_(i) denotes the signal filtered with bandpass filter i. The transition times of the signals are t=6, 30, 54, 78 mins. The ordering of the second signal is r₃, r₄, r₁, r₂, and r₃. In addition, the second signal is shifted such that the transition between filters occur at t=18, 42, 66, 90 mins. Therefore, at all times, both speakers 504 are playing sequences with different spectral content, and at no time are both speakers 504 changing their spectral content simultaneously. The speakers 504 are the external signals e_(i), and the frequency content of the external signals e_(i) is the frequency content of the signals played over the speakers 504. A spectrogram of the frequency content of both speakers 504 in shown in the upper two plots of FIG. 6.

The acoustic path in FIG. 5 is the air and the physics of the room containing the foam 502. During the experiment a foam block (not depicted) was placed in the room at time t=12 mins and then it was removed again at time t=36 mins. A plastic case (not depicted) was placed in the room in between the speakers 504 at time t=60 mins and removed at time t=85 mins. Placing objects in the room is a way to alter the acoustic path between the two sensors. Background noise was present during the collection of the data including noise from heaters, lights, outside traffic, talking in adjacent rooms, etc. The objective of the experiment was to be able to determine when the first speaker 504 a changed its frequency content, when the second speaker 504 b changed its frequency content, and finally when the acoustic path response changed, given only data obtained from the fiber optic sensors in the room. The bottom two plots of FIG. 6 show a spectrogram of the measured acoustic signals. As can be seen the measured signals change at many times, and it is not clear what has changed when only visually inspecting the measured signals' spectra.

In FIG. 7 the estimated acoustic path response is shown over the duration of the experiment. The changes at times t=24, 72, 120, 170 are very noticeable. Furthermore, the estimates appear relatively constant during the time between those changes.

In FIG. 8 the estimated frequency content of the external signals e, is plotted. Again, the changes in the signals correspond with the changes in the source, and during the times that there are no changes, the estimates appear relatively constant. FIG. 8 shows the estimated external signal frequency content does change when the acoustic channel is changed (by placing objects in the room). This is as expected by Equation (9), which shows that the estimated transfer function matrix F(q)

(q) is a function of the acoustic path response G₁₁ ^(i), G₁₂ ^(i), G₂₁ ^(i), and G₂₂ ^(i), i=1,2.

In FIG. 9 the processor 102 determines the statistical difference for the current estimate of the acoustic path response relative to the estimate 5 time blocks ago. If the acoustic path changes, and remains constant for at least 5 time blocks, the processor 102 depicts this as a dark vertical line having a width of 5 time blocks in the plot. The wide vertical lines in the plot accordingly match the times when the acoustic path was changed. In addition there do not appear to be any other vertical lines in the plot, which means that the acoustic channel was constant between the changes. By comparing FIGS. 6 and 9 it appears that the statistical difference provides a clearer indication of when the acoustic path significantly changed.

In FIG. 9 the processor 102 determines the statistical difference for the current estimate of the frequency content of the external signals to the estimate 5 time blocks ago. If the frequency content of the external signals changes, and remains constant for at least 5 time blocks, the processor 102 displays this as a dark vertical line of width 5 time blocks in the plot. This is the case when the speakers 504 a,b change their frequency content. On the other hand if the frequency content of the external signals e, changes only for a short time (<1 time block), this shows up as 2 vertical lines of width 1 time block each, spaced 5 time blocks apart. This is the case when a person walked into the room to place or remove an object.

The embodiments have been described above with reference to flowcharts and block diagrams of methods, apparatuses, systems, and computer program products. In this regard, the flowchart and block diagram in FIGS. 1A, 3, 4, and 10 illustrate the architecture, functionality, and operation of possible implementations of various embodiments. For instance, each block of the flowcharts and block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). In some different embodiments, the functions noted in that block may occur out of the order noted in those figures. For example, two blocks shown in succession may, in some embodiments, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. Some specific examples of the foregoing have been noted above but those noted examples are not necessarily the only examples. Each block of the block diagrams and flowcharts, and combinations of those blocks, may be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

Each block of the flowcharts and block diagrams and combinations thereof can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions or acts specified in the blocks of the flowcharts and block diagrams.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions that implement the function or act specified in the blocks of the flowcharts and block diagrams. The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions that execute on the computer or other programmable apparatus provide processes for implementing the functions or acts specified in the blocks of the flowcharts and block diagrams.

As will be appreciated by one skilled in the art, embodiments of the technology described herein may be embodied as a system, method, or computer program product. Accordingly, these embodiments may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware that may all generally be referred to herein as a “circuit,” “module,” or “system.” Furthermore, embodiments of the presently described technology may take the form of a computer program product embodied in one or more non-transitory computer readable media having stored or encoded thereon computer readable program code.

Where aspects of the technology described herein are implemented as a computer program product, any combination of one or more computer readable media may be utilized. A computer readable medium may comprise a computer readable signal medium or a non-transitory computer readable medium used for storage. A non-transitory computer readable medium may comprise, for example, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination thereof. Additional examples of non-transitory computer readable media comprise a portable computer diskette, a hard disk, RAM, ROM, an erasable programmable read-only memory (EPROM or flash memory), a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination thereof. As used herein, a non-transitory computer readable medium may comprise any tangible medium that can contain, store, or have encoded thereon a program for use by or in connection with an instruction execution system, apparatus, or device. Thus, computer readable program code for implementing aspects of the embodiments described herein may be contained, stored, or encoded on the computer readable medium 104 of the signal processing device 118.

Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, radiofrequency, and the like, or any suitable combination thereof. Computer program code for carrying out operations comprising part of the embodiments described herein may be written in any combination of one or more programming languages, including an object oriented programming language and procedural programming languages. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (e.g., through the Internet using an Internet Service Provider).

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. Accordingly, as used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and “comprising,” when used in this specification, specify the presence of one or more stated features, integers, steps, operations, elements, and components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and groups. Directional terms such as “top”, “bottom”, “upwards”, “downwards”, “vertically”, and “laterally” are used in the following description for the purpose of providing relative reference only, and are not intended to suggest any limitations on how any article is to be positioned during use, or to be mounted in an assembly or relative to an environment. Additionally, the term “couple” and variants of it such as “coupled”, “couples”, and “coupling” as used in this description are intended to include indirect and direct connections unless otherwise indicated. For example, if a first device is coupled to a second device, that coupling may be through a direct connection or through an indirect connection via other devices and connections. Similarly, if the first device is communicatively coupled to the second device, communication may be through a direct connection or through an indirect connection via other devices and connections.

One or more example embodiments have been described by way of illustration only. This description is been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope of the claims. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the claims. In construing the claims, it is to be understood that the use of a computer to implement the embodiments described herein is essential at least where the presence or use of computer equipment is positively recited in the claims. 

The invention claimed is:
 1. A method for determining whether an acoustic event has occurred along a fluid conduit having acoustic sensors positioned there along, the method comprising: (a) determining, using a processor and for each of the sensors: (i) a linear relationship between a measured acoustic signal measured using the sensor and a white noise acoustic source located along a longitudinal segment of the fluid conduit overlapping the sensor, wherein the longitudinal segment is delineated by a pair of fiber Bragg gratings located along an optical fiber and tuned to substantially identical center wavelengths; and (ii) from the linear relationship, an acoustic path response and an acoustic source transfer function that transforms the white noise acoustic source; (b) monitoring over time variations in one or both of the acoustic path responses and acoustic source transfer functions; (c) determining whether at least one of the variations exceeds an event threshold; and (d) when at least one of the variations exceeds the event threshold, attributing the acoustic event to one of the sensors corresponding to the acoustic path response or acoustic source transfer function that varied in excess of the event threshold.
 2. The method of claim 1 wherein the processor attributes the acoustic event to the one of the sensors for which the variation most exceeds the event threshold.
 3. The method of claim 1 wherein the acoustic event comprises one of multiple acoustic events, and wherein the processor attributes one of the acoustic events to each of the sensors for which the variation exceeds the event threshold.
 4. The method of claim 1 wherein the acoustic path response comprises an acoustic response of the longitudinal segment and the acoustic event is identified as having occurred along the longitudinal segment corresponding to the sensor to which the acoustic event is attributed.
 5. The method of claim 4 wherein, for each of the channels, the processor determines the linear relationship between the measured acoustic signal, the white noise acoustic source located along the longitudinal segment, and white noise acoustic sources located along any immediately adjacent longitudinal segments.
 6. The method of claim 4 wherein each element of the linear relationship is a parameterized transfer function that is parameterized using a finite impulse response structure.
 7. The method of claim 4 wherein the processor determines the acoustic path responses and acoustic source transfer functions by factoring the linear relationship using a linear regression, wherein the linear regression is factored into a first array of parameterized transfer functions for determining the acoustic path responses and a second array of parameterized transfer functions for determining the acoustic source transfer functions.
 8. The method of claim 7 wherein each of the first and second arrays is parameterized using a finite impulse response structure.
 9. The method of claim 4 further comprising, prior to monitoring variations in one or both of the acoustic path responses and acoustic source transfer functions, refining the one or both of the acoustic path responses and acoustic source transfer functions using weighted nullspace least squares.
 10. The method of claim 4 wherein (b)-(d) comprise: (a) determining a confidence bound for each of: (i) two of the acoustic path responses; or (ii) two of the acoustic source transfer functions; (b) from the confidence bounds, determining a statistical distance between the two of the acoustic source responses or the two of the acoustic source transfer functions; (c) comparing the statistical distance to the event threshold; and (d) identifying the acoustic event as having occurred when the statistical distance exceeds the event threshold.
 11. The method of claim 4 further comprising dividing the measured acoustic signal into blocks of a certain duration prior to determining the linear relationship.
 12. The method of claim 4, further comprising optically interrogating the optical fiber in order to obtain the measured acoustic signal.
 13. The method of claim 12 wherein the optical fiber extends parallel to the fluid conduit.
 14. The method of claim 12 wherein the optical fiber is wrapped around the fluid conduit.
 15. The method of claim 13 wherein the optical fiber is within a fiber conduit laid adjacent the fluid conduit.
 16. The method of claim 4 wherein the fluid conduit comprises a pipeline.
 17. A system for detecting whether an acoustic event has occurred along a fluid conduit longitudinally divided into measurements channels, the system comprising: (a) acoustic sensors comprising an optical fiber extending along the conduit, the optical fiber comprising fiber Bragg gratings (FBGs), wherein each of the measurement channels is delineated by a pair of the FBGs tuned to substantially identical center wavelengths; (b) an optical interrogator optically coupled to the optical fiber and configured to optically interrogate the FBGs and to output an electrical measured acoustic signal; and (c) a signal processing unit comprising: (i) a processor communicatively coupled to the optical interrogator; and (ii) a non-transitory computer readable medium communicatively coupled to the processor, wherein the medium has computer program code stored thereon that is executable by the processor and that, when executed by the processor, causes the processor to perform a method comprising: (1) determining, using the processor and for each of the sensors: a) a linear relationship between a measured acoustic signal measured using the sensor and a white noise acoustic source located along a longitudinal segment of the fluid conduit overlapping the sensor; and b) from the linear relationship, an acoustic path response and an acoustic source transfer function that transforms the white noise acoustic source; (2) monitoring over time variations in one or both of the acoustic path responses and acoustic source transfer functions; (3) determining whether at least one of the variations exceeds an event threshold; and (4) when at least one of the variations exceeds the event threshold, attributing the acoustic event to one of the sensors corresponding to the acoustic path response or acoustic source transfer function that varied in excess of the event threshold.
 18. The system of claim 17 wherein the optical fiber extends parallel to the fluid conduit.
 19. The system of claim 17 wherein the optical fiber is wrapped around the fluid conduit.
 20. The system of claim 17 further comprising a fiber conduit adjacent the fluid conduit, wherein the optical fiber extends within the fiber conduit.
 21. The system of claim 17 wherein the fluid conduit comprises a pipeline.
 22. A non-transitory computer readable medium having stored thereon computer program code that is executable by a processor and that, when executed by the processor, causes the processor to perform a method for determining whether an acoustic event has occurred along a fluid conduit having acoustic sensors positioned therealong, the method comprising: (a) determining, using a processor and for each of the sensors: (i) a linear relationship between a measured acoustic signal measured using the sensor and a white noise acoustic source located along a longitudinal segment of the fluid conduit overlapping the sensor, wherein the longitudinal segment is delineated by a pair of fiber Bragg gratings located along an optical fiber and tuned to substantially identical center wavelengths; and (ii) from the linear relationship, an acoustic path response and an acoustic source transfer function that transforms the white noise acoustic source; (b) monitoring over time variations in one or both of the acoustic path responses and acoustic source transfer functions; (c) determining whether at least one of the variations exceeds an event threshold; and (d) when at least one of the variations exceeds the event threshold, attributing the acoustic event to one of the sensors corresponding to the acoustic path response or acoustic source transfer function that varied in excess of the event threshold. 